Many information transmission problems in the radio phone system involve time-variant or statistical sources of signal degradation in a radio channel. An advantage of the digital radio telephone system compared with the analog one is that it can be designed to monitor the channel and be adapted to the changes thereof. Any transmission channel, be it a transmission line or a radio channel, affects the amplitude of the waveform, frequency or phase of the signal, thus producing intersymbol inteference among the bit pulses. In a mobile station, like a radio phone in a car, the characteristics of the channel change constantly over time. A general solution in digital cellular systems known in the art is to use adaptive channel correction. This means that certain distortion characteristics of a channel are measured periodically or continuously, and the predicted distortions in the transmitted pulses are subtracted from the received waveform. The system is capable of monitoring the quality of the channel by measuring the bit error ratio and/or other parameters, such as signal strength and delays.
The subscriber device of the cellular system may mean, depending on the system, a so-called mobile station, i.e. mobile communicating equipment, between the antenna and the base station whereof being provided a radio channel, or it may refer to a phone which is by means of a wire connection connected with a transmitter/receiver at a remote distance, between the antenna whereof and the antenna of the base station being provided a radio channel. Reference is made below primarily to a mobile communicating equipment, though it is useful to note that the same features are applicable to the subscriber device of the latter definition as well. The signal strength and the delay are bound to the signal propagation distance between the base station and the mobile station. As is well known in the art, the transmission rate is high because of the TDMA-transmission used in digital systems so that the multiple-path propagation characteristic of the radio path is visible in the reception not only in the form of rapid so-called Rayleigh fading of the envelope of the RF signal but also as an intersymbol interference between the detected bits. In view of the intersymbol interference, the signal propagation model has been so expanded in the digital systems that a received signal is no longer an individual Rayleigh-faded signal but a sum of independently Rayleigh-fading signals and signals including a different delay.
The impulse response of a radio channel can be illustrated in the time domain by means of tap presentation as shown in FIG. 1. Therein, the height of an individual tap illustrates the average strength of a Rayleigh-faded signal and the location of the tap illustrates the transmission delay. The distribution of the taps is dependent on the power levels used and the environment conditions, and the fading frequency of the taps is dependent on the speed of the mobile station, e.g. of a car. In various systems, some of such propagation models have been defined in order to illustrate various environments and vehicle velocities.
On the basis of what is said in the foregoing it is obvious that since the radio channel changes rapidly, the intersymbol interference of the detected bits caused by signal transmission across the radio channel must be corrected by measuring the impulse response of the channel and by adapting the receiver to the tap configuration of the channel. This is usually carried out in the systems so that the base station or the mobile communicating equipment transmits a known bit configuration in the transmission burst thereof, i.e. a constant-length sequence of consecutive bits. The sequence is called a training sequence. The receiver has earlier received an encoded piece of information about what kind of bit pattern, that is training sequence, will be transmitted. The receiver correlates with the prior art training sequence corresponding to the training sequence it received and equivalent to the encoded data accessed from the memory. As a result of the correlation, an estimate on the radio path (i.e. delay) is received and the receiver sets the channel equalizer so that the delay distributions are corrected on a given length. For instance, in the GSM system the delay distributions are corrected up to 16 .mu.s.
One TDMA frame, for instance in the GSM system, comprises eight time intervals. The signal is transmitted in the form of bursts, of which a so-called standard burst is shown in FIG. 2. It consists of first three tail bits, whereafter 58 data bits follow, said bits containing data or speech. They are followed by a training sequence of length of 26 bits, then again followed by 58 data bits, and finally, by three tail bits. Between the time intervals of the frame a 8.25 sec a guard period is provided. As shown in the figure, the training sequence is located in the middle of a burst as a uniform sequence, its constant length being 26 bits. Eight training sequences differing in bit configuration are provided, and pre-information has been transmitted to the phone about the type of training sequence to be transmitted by the base station.
The training sequence need not be located in the middle of a burst. Therefore, in a digital radio phone system used in the U.S.A. a frame consists of six time intervals, each containing 162 symbols. One symbol may comprise 2 bits, as in the QPSK modulation used in said system, or even more bits, depending on the modulation system. In a burst to be transmitted from a base station to a mobile station, a transmission time interval always contains first a 14 symbol synchronization burst used as a training sequence. Let it be noted that the length of a training sequence is constant. In said system six different sequences of training sequences are provided.
Let it be noted that the training sequence is transmitted both from the subscriber device to a base station (Up Link) and from the base station to the subscriber device (Down Link). The symbol sequences of the training sequences need not necessarily be the same in both directions. Whatever the system, endearours are made to provide such sequences of training sequences that they are provided with as good autocorrelation properties as possible, i.e. on both sides of a peak in the middle of an autocorrelation function a sufficient amount of zeroes are provided. A given training sequence is appropriate for a given environment. For instance in city areas the multiple path propagation of a signal is dominating, and the training sequence can be therefore different from that in the countryside where few obstacles causing signal reflections exist. In systems currently used the length of a training sequence is the constant length typical of the system and it has been selected according to the so-called worst case, whereby it is necessary to be prepared to correct the delay distortion time-wise on a long distance and it is assumed that the impulse response of the channel is multiple-tap type.
FIG. 3 shows the design of a typical training sequence. The example is selected from the GSM system. The training sequence comprises a reference part, on both sides whereof being an additional part. The length of the reference part is 16 bits, and the length of each guard part is 5 bits. The shape of a training sequence ie thus 5+16+5. FIG. 4 presents the bit sequences included in the training sequences used. As mentioned above, the sequences have been so selected that they are provided with good autocorrelation properties. The length of the guard part determines how long impulse response in said training sequence is estimatable. In the present training sequence a six-tap impulse response can be estimated. The length of the additional part in GSM has been selected to conform to the worst instance, i.e. training sequences similar in configuration are used all over the network, although not all six taps need to be estimated: if the delay distribution is small, as it is in the countryside, estimation of only a few taps would be enough.
The guard part need not be located on both sides of the reference part, such as in the GSM system, instead, it can be only one guard part which is located before or after the reference part. In practice, the guard part is so produced that the first and/or last symbols are selected for the symbols thereof.
The length of the additional parts of the training sequence and the reference part has an essential significance: the longer the reference part (the more bits or symbols), the better channel estimate is obtained because when using a long reference part the noise becomes averaged, thus not distorting the result. On the other hand, the longer is the guard part (as symbols or bits), the longer bit distributions can be measured. Now, reservations have been made e.g. in the GSM system against the most difficult instance by setting 5 bits for the length of the additional part, whereby a six-tap impulse response can be estimated.
Setting the reference part and the guard part fixed in length involves certain drawbacks. If the multipath propagation is insignificant, i.e. the impulse response of the channel is short in duration, it is of no use to utilize a long additional part, and instead, a long reference part would be preferred, whereby a better estimate of the radio channel than today could be obtained. Thus, in areas where no obstacles exist to a disturbing degree, a good quality of the connection could be provided also in long distances. On the other hand, in areas where the multiple-path propagation is dominant, it would be better to use as long as additional part as possible, whereby a multiple-tap impulse response would be provided and a channel equalizer can be disposed to correct the delay distribution time-wise on a great distance. In favourable conditions the length of the entire training sequence need not be very long. Now, capacity would be released in the burst to transfer more speech and data information.